1. Technical Field
The invention relates to a dispensing device. More particularly, the invention relates to a portable self-contained pneumatic gun for dispensing metered and mixed plural component flowable materials. Specifically, the invention relates to a portable dispensing pneumatic gun for plural component liquids in which the plural components are accurately metered and mixed adjacent to the point of dispensing.
2. Background Information
An ever increasing number of products used in everyday life require the dispensing of liquid or semi-liquid flowable materials in one form or another for their manufacture. These flowable materials typically comprise two component reactive resins. The types of materials dispensed include virtually any flowable liquid, semi-liquid, or paste such as epoxies, polyurethanes, silicones, polyester, acrylics, polysulfides and phenolics, for example. Common commercial manufacturing processes in which such materials are used include injecting precise amounts of mixed resins into molds, encapsulating electric components with insulating resins, applying continuous beads of structural adhesives, injecting polyester into closed molds, sealing joints with two part polysulfides, and numerous other functions requiring the accurate control, delivery and mixing of two reactive component materials. Examples of product application include under the hood electronic assemblies and safety devices for the automotive and trucking industry, encapsulation of magnetic and other advanced electrical devices for the air and space industry, component mounting, security potting and gun type applications for circuit board assemblies, and components and apparatus such as switches, power supplies, heating assemblies, and other electronic components for the appliance industry.
Thus, as the aforesaid flowable mixed materials continue to be consumed in increasing quantities, the demand for precise liquid and semi-liquid metering, mixing and dispensing devices is also growing at an accelerated rate. The industry is continuously searching for more reliable, efficient and accurate metering, mixing and dispensing device for plural component flowable materials for a variety of purposes. For example, a particular application may require that a device efficiently and accurately dispenses such plural materials ranging in amounts from less than 1 cubic centimeter to many gallons. However, although the industry is calling for more exact device, it is also requiring that the device design be simple, and capable of being operated by production personnel or conveniently integrated with automation devices such as robots and conveyor systems. Problems currently exist because many prior art plural component metering, mixing and dispensing device are immobile, requiring that the work be brought to the device which most often is inefficient and impractical.
Prior art metering, mixing and dispensing devices for two component liquid materials such as shown in U.S. Pat. No. 5,092,492, require the use of check values usually of the spring-biased ball detent arrangement, for controlling the flow of the two components through their respective discharge openings. Although these prior art dispensing devices using check valves are satisfactory for certain applications, they have certain drawbacks, especially when used with a high viscosity liquid and higher pressure. Likewise, with certain types of particle filled liquids, the particles could cause problems by eventually fouling and effecting the operation of the spring-biased ball check valves. Furthermore, such check valves when used in pairs, are difficult to accurately control the liquid being dispensed, especially when used in very small shot applications, such as for one cubic centimeter, since the proportion of the two liquids must be extremely accurate to achieve the desired reaction when the two components are mixed. Furthermore, the use of spring-biased ball check valves do not provide any "snuff back" effect which is desirable in certain applications to prevent excess material from dropping from the discharge nozzle. Thus, it is desirable that a positive pressure dispensing device be utilized which does not rely on check valves for their operation. However, such positive dispensing devices have only been used for the discharge of a single amount of a liquid and not for the simultaneous discharge of two components in extremely accurate amounts which are then subsequently mixed and dispensed.
One type of single liquid positive pressure dispensing device is described in U.S. Pat. No. 3,806,084, issued to R. J. Seese and entitled "Improved Valve Dispensing Apparatus". The Seese patent describes a dispensing device comprising a housing having a tubular chamber which communicates with the material inlet and exit port, and a unitary valve member defined in the passageway. The valve member is slidable in the chamber. The Seese patent describes a wiping action on the chamber by the valve member during its return stage, which causes a suction force on undispensed material, thereby preventing the sauter from dripping out of the exit port.
U.S. Pat. No. 4,095,722 issued to K. L. Miller entitled "Dripless Dispenser and Methods of Dispensing a Flowable Material", describes another type of positive pressure dispenser for a single liquid, using a bore and a pinch off tube fitted within the bore. A single flowable material reservoir is connected to one end of the pinch off tube, and a dispensing nozzle or needle is removably coupled to the other end of the tube. The pinch off tube is squeezed to dispense a shot of single flowable material. Release of the squeezed pinch off tube causes the residual material in the pinch off tube to be drawn back away from the dispensing point.
U.S. Pat. No. 4,942,984, issued to K. L. Miller entitled "Dripless Sauter Paste Dispenser" describes another single liquid positive displacement dispensing gun, using three pressure chambers wherein pressure is altered in the pistons to achieve a three stage operating system. Specifically, the resting stage is characterized by having the reservoir tube opened. To dispense a shot of semi-solid material, the positive pressure in the upper pressure chamber is reduced. The middle pressure chamber is not yet pressurized and the reduction in the upper chamber pressure enables the positive pressure in the lower chamber to force the lower piston upward, which in turn mechanically pushes the reciprocating drive rod and dispensing rod tip upward. This movement then allows semi-solid material to flow into the dispensing tube. Thereafter, the middle and upper chambers are then pressurized and the reciprocating drive rod travels downward for a predetermined distance coincident with the long axis of the housing. This in turn pushes the dispensing rod tip down to dispense a precise amount of a single component semi-solid material through the outlet port.
Another type of positive displacement system for dispensing a single fluid is described in Publication SCM/Dispensit dated 1990 and distributed by SCM Metal Products, Inc. identified by its Models: 1,000 series. This dispenser provides for the positive dispensing of a single shot of material, which although satisfactory for its intended purpose, does not enable the simultaneous dispensing matering and mixing of two components as required by the device of the present invention.
Another problem with dual component metering, mixing and dispensing devices is that the starting and stopping of the flow of both liquids simultaneously is critical in order to achieve the required uniform mixing which is essential for many applications. This is also a problem at the start-up of a dispensing operation due to the formation of air bubbles and pockets in the dispensing lines and chambers. Therefore, there is a need to be able to purge such air from the device at start-up to ensure that the initial shots of material are accurately mixed to prevent the wasting of materials or formation of improper shots of the dual liquids at start-up.
Moreover, the design of many types of the prior art metering, mixing and dispensing device, due to their bulky nature and the inability to position the apparatus in close proximity to the worker, require lengthy hoses for transport of the metered and/or mixed material, the components of which often begin to react prematurely, sometime before it is actually dispensed, which is highly undesirable. Rather, it is preferable that the dual flowable materials be metered and mixed as closely as possible to the point of dispensation or application to avoid premature reaction of the materials. Also, locating the metering and mixing components of the device as closely as possible to the dispensing point increases metering accuracy and control.
Another problem with known dual component dispensing guns is the matter of overrun discharge of the flowable materials when dispensation is stopped or terminated. The slow release of pressure on a piston member causes the materials to continue to flow at a decreased rate until pressure is fully relieved resulting in inaccurate dispensing and improper ratios of the mixed materials. Where two component materials are dispensed simultaneously, they may have different flow and viscosity characteristics accentuating the inaccuracy of desired delivery. Loss of precise delivery of desired amounts is a frequent problem, especially where small volumes are dispensed, and when spring-biased ball check valves are utilized as in known dual liquid dispensing equipment.
In the past, the deposition of adhesives, sealants, lubricants and the like has been plagued by other problems. In the absence of any type of a positive displacement mechanical dispenser, the application of such materials is more often than not a messy and inaccurate operation. Frequently, an expensive substance is haphazardly applied, wasting valuable material and generating unnecessary cleanup costs. Even the use of one of the many types of dispensers heretofore known to those skilled in the art has failed to eliminate all of the problems. While many of these dispensing devices may dispense certain materials accurately, they are still not capable of producing uniform shots of a two component flowable material, the viscosities of which are subject to change, a common phenomenon in flowable materials such as epoxy resin adhesives, nor can they accommodate a variety of materials having a wide range of viscosities. Conventional dispensers may reduce the waste material, but the necessary periodic readjustment of these dispensing devices produces undesirable "down-time", creating inefficiencies in a common production process situation.
Therefore, the need exists for an improved liquid metering, mixing and dispensing device in which plural component materials are metered and mixed adjacent to the point of dispensing thereof, and which device is portable enough to be handled by a human operator or easily integrated with automated systems. Moreover, the need also exists for an improved liquid metering, mixing and dispensing machine wherein overrun discharge is controlled as pressure is removed from the discharge tip immediately after discharge, while simultaneously the reservoir inlet is occluded during the dispensing operation. Further, the need exists for such a device wherein a plurality of materials may be simultaneously metered, at different rates, wherein an exact predetermined amount of each material may be dispensed into a mixing head.